While active plasmonics in the ultraviolet to visible has been demonstrated, control in the near-infrared (NIR) to mid-infrared (MIR) spectral range has recently attracted significant attention for importance in telecommunications, thermal engineering, infrared sensing, light emission and imaging. Infrared plasmonics has been demonstrated with materials such as colloidal quantum dots, Si and InAs, and graphene. However, challenges include material instabilities and strong infrared absorption of solvents (quantum dots in solution), limited carrier densities (Si and InAs) and spectral range (graphene).
Noble metallic nanostructures possess large negative permittivity in the visible and near-infrared (NIR) range, and can therefore concentrate optical fields into subwavelength dimensions with enhanced nonlinear plasmonic response. However, the high electron concentration in noble metals limits the extent to which the electron distribution can be modified and with it the achievable permittivity modulation. In addition, strong interband transitions in the visible range (such as those from the d-band to the Fermi-surface in gold at an energy of ˜2.4 eV) give rise to a large dispersion of the permittivity modulation versus wavelength, which furthermore can overlap with their plasmonic resonances, thereby complicating the design of nonlinear optical devices.
As a result, there is a need for new materials and methods for all-optical modulation over a broad spectral range (from the visible to the infrared range) with ultrafast dynamics.